1. Field of the Invention
Preferred example relates to the art of a selectable one-way clutch adapted to selectively enable torque transmission only in one direction and to interrupt torque transmission in both directions.
2. Discussion of the Related Art
Japanese Patent Publication No. 5120650 describes a hybrid drive unit having one-way clutches. According to the teachings of Japanese Patent Publication No. 5120650, power of an engine is distributed to a first motor-generator and to an output member through a planetary gear unit serving as a power distribution device. The first motor-generator is operated as a generator in such a manner to establish a reaction force to control a speed of the engine, and a second motor-generator is operated as a motor by an electric power generated by the first motor-generator to generate a driving torque. The one-way clutches are disposed between the first motor-generator and a stationery member such as a casing, between the first motor-generator and a carrier of the planetary gear unit, and between the carrier and the stationary member. Those one-way clutches are arranged in tandem with dog clutches. In the hybrid drive unit taught by Japanese Patent Publication No. 5120650, therefore, rotation of a sun gear or the carrier of the planetary gear unit connected to the first motor-generator can be halted in a predetermined direction so that the sun gear and the carrier can be rotated integrally to diversify a driving condition.
A conventional selectable one-way clutch is adapted to selectively enable torque transmission only in one direction. To this end, in the selectable one-way clutch, a selector plate interposed between a pocket plate and a notch plate is rotated by an electromagnetic actuator or the like to selectively provide an engagement between the selector plate and the pocket plate. The selectable one-way clutch of this kind may also be used in the hybrid vehicle taught by Japanese Patent No. 5120650 instead of the one-way clutch.
In order to reduce friction, lubrication oil is applied to the selectable one-way clutch to form oil films in friction sites between the pocket plate and the selector plate, and between the selector plate and the notch plate. However, if viscosity of the oil is too high, the selector plate may be rotated undesirably by a drag torque or a shearing force resulting from a rotation of the notch plate if the pocket plate is halted. For this reason, the pocket plate may be brought into engagement accidentally with the notch plate if temperature is extremely low.